1. Field of the Invention
The present invention relates to a manufacturing method of a thin-film semiconductor formed on a substrate having an insulative surface, such as a glass substrate.
2. Description of the Related Art
In recent years, a thin-film transistor attracts attention as a semiconductor device using a thin-film semiconductor. In particular, a configuration in which thin-film transistors are incorporated in a liquid crystal electro-optical device attracts much attention. In this liquid crystal electro-optical device, a thin-film semiconductor is formed on a glass substrate that is a component of the device, and thin-film transistors are constructed by using the thin-film semiconductor. Provided for each pixel electrode of the liquid crystal electro-optical device, the thin-film transistor serves as a switching element for controlling charge entering or exiting the pixel electrode. Called an active matrix liquid crystal display device, this device can display images of very high quality.
An amorphous silicon thin film is mainly used as a thin-film semiconductor for thin-film transistors. However, at present, thin-film transistors formed by using an amorphous silicon thin film cannot provide necessary characteristics.
To improve the characteristics of an amorphous silicon film, it is effective to crystallize it into a crystalline silicon film. Among the known methods of obtaining a crystalline silicon film is a method in which an amorphous silicon film formed by plasma CVD or low-pressure thermal CVD is subjected to a heat treatment.
On the other hand, in utilizing thin-film transistors in an active matrix liquid crystal electro-optical device, there is a problem that a glass substrate needs to be used from the economical point of view.
To crystallize an amorphous silicon film by a heat treatment, it should be performed at a temperature higher than 600.degree. C. for more than several tens of hours. But such a heat treatment may warp or deform a glass substrate. This is particularly remarkable in a large-area glass substrate. Since a liquid crystal electro-optical device is constructed such that a liquid crystal is interposed and held between two glass substrates that are bonded to each other with a gap of several micrometers, deformation of the glass substrates is undesirable because it may cause unevenness in display, for instance.
This problem may be solved by using quartz substrates or special glass substrates that can endure a high-temperature heat treatment. However, since such substrates are expensive, it is difficult to use those substrates from the viewpoint of the manufacturing cost.
There is known a technique of crystallizing an amorphous silicon film by irradiating it with laser light. Although this technique is advantageous in being capable of locally forming a crystalline silicon film having very high crystallinity, it has a difficulty in providing a uniform effect of irradiation with laser light over the entire film. Further, resulting crystalline silicon films vary in quality from one process to another; in other words, this technique is low in reproducibility.